Since we don't need the actual split values, instead of calling
`strings.Split`, which allocates new slices on each call, use
`strings.Index`.
This significantly reduces the allocations required when doing env value
replacements.
Additionally, pre-allocate the env var slice, even if we allocate a
little more than we need, it keeps us from having to do multiple
allocations while appending.
```
benchmark old ns/op new ns/op delta
BenchmarkReplaceOrAppendEnvValues/0-8 486 313 -35.60%
BenchmarkReplaceOrAppendEnvValues/100-8 10553 1535 -85.45%
BenchmarkReplaceOrAppendEnvValues/1000-8 94275 12758 -86.47%
BenchmarkReplaceOrAppendEnvValues/10000-8 1161268 129269 -88.87%
benchmark old allocs new allocs delta
BenchmarkReplaceOrAppendEnvValues/0-8 5 2 -60.00%
BenchmarkReplaceOrAppendEnvValues/100-8 110 0 -100.00%
BenchmarkReplaceOrAppendEnvValues/1000-8 1013 0 -100.00%
BenchmarkReplaceOrAppendEnvValues/10000-8 10022 0 -100.00%
benchmark old bytes new bytes delta
BenchmarkReplaceOrAppendEnvValues/0-8 192 24 -87.50%
BenchmarkReplaceOrAppendEnvValues/100-8 7360 0 -100.00%
BenchmarkReplaceOrAppendEnvValues/1000-8 64832 0 -100.00%
BenchmarkReplaceOrAppendEnvValues/10000-8 1146049 0 -100.00%
```
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
